a) Field of the Invention
The invention is directed to a method for reducing color moiré in digital images.
b) Description of the Related Art
When recording images with digital cameras, high-frequency signals are scanned at a fixed frequency given by the pixel raster of an image sensor. In large-area, finely structured regions, the sampling points often produce low-frequency beats which result in color moiré interference. The wavelengths of these beats can amount to a considerable multiple of the pixel distance and are highly dependent upon the recording situation.
Numerous technical solutions have been disclosed that relate to the reduction of moiré effects brought about in digital images by undersampling. These include solutions that prevent structures causing moiré from being transmitted to the image sensor such as optical filters by which the sharpness is removed from fine image structures before they are imaged on the image sensor, so that the high-frequency signals are absent during scanning (DE 37 89 291 T2). Also, a deliberately generated movement blur (JP 04 061 587 A) leads to a result of this kind, but image information is lost and the image sharpness is reduced in both cases.
Another possibility consists in providing an asymmetric color distribution in the pixel raster in that the color distribution does not repeat at constant intervals on the pixel matrix. Accordingly, the scanning conditions are varied and the moiré effects are reduced (U.S. Pat. No. 4,821,088).
Finally, there are known techniques which act on the image that has already been scanned in that the digital image is first transformed into a luminance/chrominance color space and then the upper frequencies of the chrominance signals are filtered out. After filtering, the image is converted back into the original RGB color space. It is disadvantageous that no distinction is made between a color moiré and actual color image signals. The occurrence of color distortions in the edge areas becomes more pronounced as the signal intensity in the channels is decreased for reducing the color moiré.
EP 1 083 737 A2 also does not succeed in adequately eliminating these color distortions because, through the exclusive use of hue values for localizing the regions in which color moiré is present, important signal characteristics of moiré phenomena are not taken into account and the moiré intensity itself is not taken into account during the color change for the localized moiré pixels by means of hue values that are averaged over a large area.